1. Field of the Invention
The prevent invention relates to a magnetic resonance imaging (MRI) system for medical diagnosis and a noise insulating method to be implemented in the system. More particularly, this invention is concerned with greatly suppressing noises derived from the pulsating drive of a gradient coil unit and with a noise insulating method to be implemented in the MRI system.
2. Description of the Related Art
A magnetic resonance imaging system for medical diagnosis is an imaging system utilizing a magnetic resonance phenomenon exhibited by nuclear spins in a subject body. The magnetic resonance imaging system is non-invasive and can produce images of the inside of a subject in the absence of exposure to X rays. The usefulness of the magnetic resonance imaging system even in clinical practice has been proven.
In general, a magnetic resonance imaging system for producing MR images comprises a gantry having a diagnostic space in which a subject is inserted and positioned, and a main unit interlocked with the gantry. The gantry is provided with various components, e.g., a magnet for generating a static magnetic field in the diagnostic space, such as, a superconducting magnet, a gradient coil unit for generating linear magnetic field gradients to be supposed on the static magnetic field, and a radio-frequency (RF) coil for transmitting a radio-frequency signal and receiving an MR signal are indispensable. For scanning, the magnet, gradient coil unit, and RF coil are driven according to a desired pulse sequence. Namely, linear magnetic field gradients changing in strength in x-axis, y-axis, and z-axis directions are superposed on a subject lying in the static magnetic field. Nuclear spins in the subject are excited magnetically by a radio-frequency signal at the Larmor frequency. A magnetic resonance (MR) signal produced by the excitation is detected. For example, a two-dimensional tomographic image of the subject is reconstructed on the basis of the signal.
In the above magnetic resonance imaging, the need to speed up imaging (i.e., to shorten the time required for imaging has increased in recent years. For coping with the need, an imaging technique using a pulse sequence including a gradient pulse to be switched (reversed) at a high speed; such as, fast echo planar imaging (EPI) has been developed. Some of these techniques have successfully been put to practical use. When a gradient pulse is generated, an electromagnetic force works on a gradient coil unit at the rise or reversal of the gradient pulse at audio frequencies. The electromagnetic force mechanically distorts the coil unit. This causes the whole unit to vibrate. There is a problem that acoustic vibrations occur with vibrations made by the coil unit and consequently loud acoustic noises occur. In particular, when a gradient pulse is reversed at high speed, a vibrations caused by application of the pulse are intensified. From this viewpoint, as imaging is speeded up, induced acoustic noises are intensified. The noises may give a subject (patient) lying down in the diagnostic space of the gantry a great sense of discomfort or unease.
Several proposals have been made in an effort to eliminate such noises in the past. As described in, for example. Japanese Unexamined Patent Publication Nos. 59-174746, 63-246146, 3-268743, and 6-189932 (hereinafter, the first to fourth prior arts), the whole unit of gradient coils in sealed in a vacuum container in order to discontinue acoustic propagation of vibrations or noises by utilizing a vacuum space.
However, the foregoing known anti-noise measures still have unsolved problems as described below.
The first to fourth prior arts have proposed a structure in which a gradient coil unit is merely enclosed with a vacuum space. A container and covers defining the vacuum space are mechanically connected to the cover and housing of a magnet for generating a static magnetic field, and the gradient coil unit itself is supported by the container and cover of the magnet for generating a static magnetic field. Part of the vibrations (noises) stemming from the gradient coil unit is cut off by the vacuum space but another part of the vibrations propagates along supporting units for supporting the gradient coil unit and reaches the magnet for generating a static magnetic field. The magnet for generating a static magnetic field thus also vibrates due to the vibrations made by the gradient coil unit. This poses a problem in that the whole of a gantry then serves as a source of acoustic vibrations to cause loud noises. That is to say, the known measures of enclosing the gradient coil unit within a vacuum are imperfect for suppressing noises.